Nowadays, a variety of recognized existing types of mills generally include screens, hammers, rotors, machine body and motors. During operation, the motor drives the rotors, on which the hammers are rotating and crushing the material so that acceptable sizes of pulverized particles are discharged from the screen sieve.
Such hammer mills usually have the deficiency as described below:
1. They only produce coarse material (e.g., poultry feed with screen sieves of 1, 2, 3 and 4 mm in diameters) and are incapable of producing fine material (e.g., aquatic feed with screen sieves of 40, 50, 60, 70, 80, 90, 100, 120, and 140 meshes, or more.)
2. They may cause the finished material to be uneven in particle size.
3. These types of hammer mills are inefficient.
The major reasons for inefficiency include:
A. The screening and crushing capability of the hammer mills is relatively inadequate so pulverized particles will not be discharged promptly and may cause repeated cycling of the material for processing to the acceptable sizes, with a result that the fine particle ratio is increased, uneven sizes of finished material are produced and the power consumption is more.
B. Generally, the blades of the screened hammer mill have dual functions of crushing and serving as a power source to discharge the particles through the screen sieves. The hammer blades, as the sole power source in the screening process, may be blocked by multi-layers of the material cycling near the surface, thereby limiting the push and extruding forces that are applied to the material through the screen. For coarse material with greater sizes and less layers of material cycling required, the acting forces of the hammer blades are effected on the surface of the screen, resulting in smooth discharge of the pulverized particles of the feed material whereas for the fine material, the diameters are far smaller than the clearance between the screen and the hammer and more material cycling is required, it is more difficult for the acting force of the hammer blades to reach the surfaces of the screen, thus retaining the material on the screen surface, such retention state of the material may cause blockage of the finished material. Hence such screened hammer mills are not able to process fine particles of the material with 40 to 140 meshes.
Previous improvement mades always focused the improvement on the physical factors such as screen shape design, screen construction, clearance between the hammer and the screen, aspirating system and the hammer configuration and so on, without considering the active power source with the screen and studing further. Several examples of the previous improvement are given below:
a. Improvement on shapes of the screen sieves--such as fish scaled sieves type of screens (refer to the article "Preliminary Report on the Effect of Scaled Screen Plate" as published on "Shanghai Feed" magazine Jan. 1994 in China.).
b. Improvement on extended screen surfaces--such as axially fed widening screen type of the hammer mill (refer to the article "Application on Improved Process of Model 91940-40 Economic Feed Hammer Mill" as published on "China Feed" magazine, Issue 1, 1995); and also the type of the hammer mill is shown in FIG. 4 of U.S. Pat. No. 4,114,817 (Harris) (Sep. 19, 1978) with such a feature that the surface area of the screen is increased by extending the axially screen width.
c. Further improvements are by having an increasingly rounded screen comer, increasing of screen surface and mounting method, such as vertical rotor type hammer mill with no aspiration installation (refer to the article "Application on Improved Process of Vertical Rotor Hammer Mill" as published on "China Feed" Issue 1, 1991; various axially feed designs of hammer mills; dipping type hammer mills; and also a type of hammer mill disclosed in FIG. 2 of U.S. Pat. No. 3,790,093 (Mcintyre) (May 2, 1974) has a screen development on the left side, which increases the surface areas, but the screen is considered liable to be blocked by larger size material at a section of the left side, thus failing to achieve the anticipated result. Moreover, this improvement is an eccentric feed design which fails to make full use of the forward and reverse rotation of the hammer, resulting in non-uniform wear of the hammers and the screen.
d. Improvement on aspirating system of the hammer--refer to the article "Design Improvement and Study on the Aspirating System of Hammer Mills--Non-aspirating Vertical Rotor Hammer Mill" as published on "China Feed" Issue 17, 1995.
e. Improvement on mounting with roll chain end-connected spring and hammer mill aspirating system with holes at the axial wall and air blow blades patented in China, China Patent No. CN 2156931 (Li Dayan) (Feb. 23, 1994) describes a type of vibrating screen. However, it is not a vibrating screen with an active power source, but the vibration is induced by material impacting screen surfaces with no control of vibrating parameters.
f. Improvement on clearance between the hammer and the screen--the improved clearance between the hammer and the screen of the mill is adjustable (in two steps), which is capable of processing the feed material with feed sizes of 0.8 to 4.0 mm in diameters. This improvement of hammer mill has extended the range of the material to be processed as compared to conventional mills, but is not applicable for the sizes of 60 to 140 meshes fine material.
In short, lots of improvements have been made on the hammer mills and have achieved certain results, yet no hammer mills in the world by then are capable of processing both coarse material (sieves 1 to 4 mm in diameters) and fine material sizes (40 to 140 meshes).
Abundant experiments have indicated that the improvement made on the active power source to the screen and proper selection of mounting of the special vibrating source and the vibrating parameters, e.g., vibrating direction, frequency, amplitude, etc., are the keys to solve the material blockage problems so that the screen will be no more solely effected by the acting force of the hammer blades, but rely on its independent discharging capability, which will have enough capability for screening the fine material particles.